Experimental Characterization of 3D-Printed Geopolymer Concrete with Recycled Carbon Fibers

Abstract

The construction industry faces a critical transition toward sustainable 3D Concrete Printing to enhance efficiency and minimize waste, yet the reliance on Ordinary Portland Cement (OPC) and the inherent mechanical anisotropy of extruded geopolymer layers remain significant barriers to structural integrity. This research addresses these challenges by investigating the synergistic potential of Recycled Carbon Fibers within a zero-cement geopolymer matrix, specifically aiming to enhance interlayer bond strength and reduce the anisotropic index of printed elements. Using a laboratory-based experimental framework, mixtures with rCF volume fractions of 0.0% (control), 0.5%, 1.0%, and 1.5% were fabricated via a gantry-based robotic system and subjected to rheological, mechanical, and microstructural analysis. The findings reveal that the 1.0% rCF inclusion (GEO-CF10) provided an optimal balance for 3DCP, achieving a compressive strength of 48.5 MPa (X-axis) and 44.8 MPa (Z-axis), which significantly reduced the anisotropy index from 0.365 in the control mix to 0.081. Furthermore, this mixture exhibited a 126% improvement in inter-layer tensile bond strength (4.12 MPa) and a substantial increase in flexural toughness energy compared to unreinforced specimens. The novelty of this study lies in the successful integration of industrial waste-derived rCF to establish a near-isotropic structural framework, effectively transforming brittle delamination failure into ductile structural collapse. These results provide a robust benchmark for developing high-performance, sustainable, and automated civil infrastructure, demonstrating that optimized fiber reinforcement can overcome the structural discontinuities typically associated with layer-by-layer deposition in geopolymer systems.